scmutil: introduce deprecated alias for revpair()
revsingle() returns a context object, revpair() returns nodeids,
revrange() returns integer revisions (in a revset). I'm going to
reduce this inconsistency by making revpair() return context
objects. Changing the return type is not nice to extensions, so this
patch introduces a nodeid-returning version of revpair() that they can
detect and use. Update callers to the new function so we can change
revpair() itself and then migrate them back one by one.
Differential Revision: https://phab.mercurial-scm.org/D3005
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